1. Technical Field
The present invention relates to the field of radio frequency (R.F.) output power control and more particularly to an R.F. power amplifier circuit comprising a power control loop including an R.F. power amplifier. The invention further relates to a method of controlling the output power of the R.F. power amplifier.
2. Description of the Prior Art
Modern R.F. applications like wireless communications devices require an efficient R.F. output power control for example to ensure a high transmission quality and to keep output signal fluctuations within limits that are defined in various standards.
Typically, R.F. output power control involves a power control loop including the R.F. power amplifier, a detector device for detecting the output power of the R.F. power amplifier and an error amplifier. Such an R.F. power control loop is known for example from U.S. Pat. No. 5,378,996.
The power amplifier known from U.S. Pat. No. 5,378,996 has a control input, a signal input, an R.F. power output and a detector output. The detector signal at the detector output is provided by a detector device in the form of a detector diode coupled between the R.F. power output and the detector output of the power amplifier. The detector output is coupled to a negative input of the error amplifier and an output of the error amplifier is connected to the control input of the power amplifier. Thus the power amplifier, the detector and the error amplifier form a power control loop with negative feedback. The error amplifier also has a positive input to which a reference signal is applied.
The R.F. output power control described in U.S. Pat. No. 5,378,996 is based on the measurement of the R.F. output power. However, in principle R.F. output power control could also be based on a measurement of the R.F. power amplifier current, i.e. on the power or current consumption of the R.F. power amplifier.
R.F. output power control based on a measurement of the R.F. power amplifier current is described in Ashok Bindra, “Smart biasing keeps R.F. power amplifier on track”, electronic design, 21 Jan. 2002, pp. 38, 40. In this article a monolithic controller that regulates and controls the output power of an R.F. power amplifier is described.
The controller is part of a closed loop solution that permits calibration of the R.F. power amplifier's gate biasing voltage in real-time modes. A schematic block diagram of an R.F. power amplifier circuit comprising the known controller is depicted in FIG. 1a. As becomes apparent from FIG. 1a, the R.F. power amplifier circuit 10 comprises a power control loop 12 and a signal supply branch 14. The power control loop 12 comprises an R.F. power amplifier 22, a current sensing element in the form of a resistor 24, a detector in the form of a comparator 16 and a filter 18.
The resistor 24 is used to sense the drain current of the R.F. power amplifier 22. The drain current is converted into a voltage that is fed together with an external voltage reference from the signal supply branch 14 to the comparator 16.
The output signal of the comparator 16 is filtered by the filter 18 and the filtered signal is used to control the R.F. power amplifier's 22 gate biasing voltage.
To cope with temperature drift and aging that affect efficiency and linearization of the R.F. power amplifier 22, a control input 14′ is provided outside the power control loop 12 between the signal supply branch 14 and the comparator 16. By means of a control signal applied to the control input 14′ the output power POUT of the power amplifier 22 can be controlled.
Departing from an R.F. output power control scheme taking into account the R.F. power amplifier current, there is a need for a R.F. power amplifier circuit which allows a robust implementation of a power control scheme. There is also a need for a method of controlling the R.F. power amplifier of such an R.F. power amplifier circuit.